Ephemera danica, commonly known as the Green Drake or Danica Mayfly, is a species of mayfly found across much of Europe. It is one of the largest and most distinctive mayfly species, renowned for its significant role in freshwater ecosystems and its importance to fly fishing.
Classification
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Ephemeroptera (Mayflies)
- Family: Ephemeridae
- Genus: Ephemera
- Species: Ephemera danica
Description
Ephemera danica is characterized by its relatively large size, with adult females reaching up to 25 mm in body length, and males slightly smaller. Its most distinctive features include:
- Body Coloration: The abdomen is typically yellowish-white with prominent dark brown or black markings along the dorsal side, forming a characteristic pattern of two parallel stripes on each segment.
- Wings: Transparent, net-veined wings, often with a slight yellowish or brownish tint, especially on the forewings.
- Legs: Yellowish-white, often striped with dark bands.
- Tails (Cerci): Adults possess three long, slender tails, which can be significantly longer than the body itself, especially in males.
- Nymphs: The aquatic nymphs are large, stout, and burrowing, with feathery gills along the sides of the abdomen and strong digging forelegs. Their coloration typically blends with their substrate.
Habitat and Distribution
Ephemera danica is widely distributed across Europe, from Scandinavia to the Mediterranean, and eastwards into parts of Asia. It thrives in clean, well-oxygenated freshwater environments, typically found in:
- Rivers and Streams: Particularly those with gravelly or sandy bottoms where the nymphs can burrow.
- Lakes: Often in the littoral zones of large, clear lakes.
The presence of E. danica often indicates good water quality, as the nymphs are sensitive to pollution.
Life Cycle
Like all mayflies, Ephemera danica undergoes an incomplete metamorphosis, with distinct aquatic nymph and terrestrial adult stages:
- Egg: Females deposit eggs directly into the water, often by dipping their abdomens or releasing them in flight. The eggs sink to the bottom and hatch after a period, which can vary depending on temperature.
- Nymph (Larva): The nymph stage is the longest part of the life cycle, lasting one to two years. Nymphs are burrowers, digging tunnels in the substrate of rivers and lakes using their specialized forelegs. They feed on detritus, algae, and small organic particles filtered from the water. They undergo numerous molts as they grow.
- Subimago (Dun): When mature, the nymph swims to the surface or crawls onto emergent vegetation. It then undergoes its final aquatic molt, emerging as a winged subimago. The subimago is duller in color, with opaque wings and shorter tails compared to the adult. This stage is relatively short, lasting from a few hours to a couple of days. Subimagoes fly to nearby vegetation to rest and complete their final molt.
- Imago (Spinner): The subimago molts one last time to become a sexually mature imago, or "spinner." This adult stage is short-lived, typically lasting only a day or two (hence "ephemeral"). Adults do not feed; their sole purpose is reproduction. Males engage in mating swarms, typically at dusk, attracting females. After mating, females lay their eggs, completing the cycle, and then die.
Ecological Role
Ephemera danica plays a crucial role in freshwater ecosystems:
- Food Source: Nymphs are an important food source for many freshwater predators, including fish (especially trout and grayling), amphibians, and aquatic insects. Adult mayflies are similarly preyed upon by birds, bats, and other insects.
- Bioindicator: Due to their sensitivity to water pollution, E. danica nymphs are often used as biological indicators of water quality. Their presence generally signifies a healthy, unpolluted aquatic environment.
- Detritivores: As nymphs, they help in the decomposition and recycling of organic matter by consuming detritus.
Angling Significance
Ephemera danica is highly significant in the sport of fly fishing, particularly for trout and grayling. The emergence of the subimagoes (duns) and the return of the spent imagoes (spinners) trigger intense feeding frenzies among fish. Anglers often tie artificial flies that imitate the various life stages of E. danica, such as:
- Green Drake Nymph: Imitating the burrowing nymph.
- Green Drake Dun: Imitating the emerging subimago.
- Green Drake Spinner: Imitating the spent adult after egg-laying.
The "Danica hatch" is a much-anticipated event for fly fishermen in regions where the species is abundant.